Conventionally, a liquefied gas storage tank with a double-shell structure in which a vacuum space is formed between an inner shell and an outer shell is known as a tank for an ultracold liquefied gas. For example, Patent Literature 1 discloses a liquefied gas storage tank, in which the outer side surface of an inner shell is covered with a thermal insulating film, and the inner side surface of an outer shell is covered with a leakage-prevention thermal insulating layer.
Since the ultracold liquefied gas has an extremely low boiling point, the liquefied gas comes to the boil if the degree of vacuum in the vacuum space degrades and heat is transferred from the outer shell to the inner shell. In the liquefied gas storage tank disclosed in Patent Literature 1, the leakage-prevention thermal insulating layer is provided for preventing such boiling of the liquefied gas. Specifically, even if the degree of vacuum in the vacuum space has degraded, the leakage-prevention thermal insulating layer keeps the thermal insulation of the liquefied gas storage tank to a certain degree.